There is now a well-established body of quantitative theory concerning the population genetics of transposable elements in sexual species. This theory predicts that element frequencies at chromosomal sites into which elements can insert should be low in natural populations. If selection acts to limit copy numbers, frequencies at X-linked sites will be lower than at autosomal sites. Furthermore, there will be a tendency for elements to accumulate preferentially in genomic regions where crossing over is absent. It is proposed to test these predictions in Drosophila melanogaster, using the technique of in situ hybridization of labeled probes of families of repeated DNA to polytene salivary chromosomes. Accumulation of elements with restricted crossing over will be tested by determining the abundance of elements in old-established, laboratory balanced inversion stocks, in rare naturally occurring inversions, and in regions where crossing over is normally absent such as chromosome 4 and the distal end of the X. Comparisons will be made with the abundance in suitable controls. Samples of X, 2nd, 3rd and 4th chromosomes from natural populations will be used to check the predictions of low element frequencies and a lower abundance of elements on the X relative to the autosomes.